The most comon vector- borne pathogen of humans in the USA is Borrelia burgdorferi, the Lyme disease spirochete, transmitted by the tick Ixodes scapularis. An understanding of the spirochete's biology in the tick, especially aspects related to antigenicity and infectivity for the mammal, is prerequsite to the development of effacious vaccines. Studies show that candidate outer surface protein vaccines are primarily active against the spirochete within the tick and that the spirochete is genotypically stable in the mammalian host. Thus it is in the tick where the selection of future antigenic and virulence varients of B. burgdorferi will occur and from where new strains of human pathogenic B. burgdorferi will emerge. Our objective is to elucidate the adaptive mechanisms evolved by Lyme borreliae that ensure their survival in the vector tick. Our central hypothesis is that environmental conditions and factors within the tick induce genotypic (extrachromosomal and chromosomal) and phenotypic (antigenicity and infectivity) changes in the spirochetes. The specific aims are to 1) determine the cues that induce functional and structural changes in B. burgdorferi when it is in the tick; 2) examine the mechanisms that mediate adhesion and invasion of tick cells; and 3) determine factors involved with the reacquisition of infectivity. To accomplish these goals, cloned and uncloned spirochetes in tick cell and organ cultures will be subjected to changes that simulate a tick environment, and their response determined by DNA (PCR, plasmid and restriction analysis, Southern blots) and protein analysis (SDS-PAGE), and serology (Western blot). Outer surface proteins (A,B,C,D,E, and F) will be followed using monoclonal antibodies (Mabs) and DNA probes. Immunolabeling, light and electron microscopy will be used in conjunction with specific inhibitors to find cell surface binding sites and assess the mode of invasion and penetration. Polyclonal antisera and Mabs against spirochete and tick cell epitopes, as well as extracellular matrix and adhesion molecules will be used to identify these sites. Ixodes scapularis cell - organ cultures and experimentally infected ticks will be used to address the phenomena of reacquisition of infectivity. The infectivity, induction of arthritis and protective immunity by spirochetes from various culture systems and ticks will be examined using the hamster arthritogenic strain JMNT.